I'm so happy to have found this topic!
If you're looking for classic zombies you can't go wrong with Romero films (the Dawn of the Dead remake is quite good, too)
Lucio Fulci's Zombie (aka Zombi 2; in Italy it was marketed as a Dawn of the Dead sequel)
Evil Dead (if you love ridiculous gore and wonderfully cheap effects)
If you haven't already seen them: The Exorcist, Rosemary's Baby, Psycho, The Birds, An American Werewolf in London, The Hills Have Eyes (original), and classic slasher and monster films.
One of my favorites is Wes Craven's Last House on the Left, but some find it to be quite brutal (the remake is being released March 13th).
Dario Argento and Mario Bava films are good if you're interested in Italian horror.
I'm excited that [REC], Quarantine, and The Cottage were mentioned because they're all in my Netflix queue.